xref: /freebsd-src/contrib/llvm-project/libcxx/include/semaphore (revision 5f757f3ff9144b609b3c433dfd370cc6bdc191ad) 
1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP_SEMAPHORE
11#define _LIBCPP_SEMAPHORE
12
13/*
14    semaphore synopsis
15
16namespace std {
17
18template<ptrdiff_t least_max_value = implementation-defined>
19class counting_semaphore
20{
21public:
22static constexpr ptrdiff_t max() noexcept;
23
24constexpr explicit counting_semaphore(ptrdiff_t desired);
25~counting_semaphore();
26
27counting_semaphore(const counting_semaphore&) = delete;
28counting_semaphore& operator=(const counting_semaphore&) = delete;
29
30void release(ptrdiff_t update = 1);
31void acquire();
32bool try_acquire() noexcept;
33template<class Rep, class Period>
34    bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
35template<class Clock, class Duration>
36    bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);
37
38private:
39ptrdiff_t counter; // exposition only
40};
41
42using binary_semaphore = counting_semaphore<1>;
43
44}
45
46*/
47
48#include <__assert> // all public C++ headers provide the assertion handler
49#include <__atomic/atomic_base.h>
50#include <__atomic/atomic_sync.h>
51#include <__atomic/memory_order.h>
52#include <__availability>
53#include <__chrono/time_point.h>
54#include <__config>
55#include <__thread/poll_with_backoff.h>
56#include <__thread/timed_backoff_policy.h>
57#include <__threading_support>
58#include <cstddef>
59#include <limits>
60#include <version>
61
62#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
63#  pragma GCC system_header
64#endif
65
66#ifdef _LIBCPP_HAS_NO_THREADS
67# error "<semaphore> is not supported since libc++ has been configured without support for threads."
68#endif
69
70_LIBCPP_PUSH_MACROS
71#include <__undef_macros>
72
73#if _LIBCPP_STD_VER >= 14
74
75_LIBCPP_BEGIN_NAMESPACE_STD
76
77/*
78
79__atomic_semaphore_base is the general-case implementation.
80It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
81functions. It avoids contention against users' own use of those facilities.
82
83*/
84
85#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
86
87class __atomic_semaphore_base
88{
89    __atomic_base<ptrdiff_t> __a_;
90
91public:
92    _LIBCPP_HIDE_FROM_ABI
93    constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a_(__count)
94    {
95    }
96    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
97    void release(ptrdiff_t __update = 1)
98    {
99        auto __old = __a_.fetch_add(__update, memory_order_release);
100        _LIBCPP_ASSERT_UNCATEGORIZED(__update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
101
102        if (__old > 0)
103        {
104            // Nothing to do
105        }
106        else if (__update > 1)
107            __a_.notify_all();
108        else
109            __a_.notify_one();
110    }
111    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
112    void acquire()
113    {
114        auto const __test_fn = [this]() -> bool {
115            auto __old = __a_.load(memory_order_relaxed);
116            return (__old != 0) && __a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
117        };
118        __cxx_atomic_wait(&__a_.__a_, __test_fn);
119    }
120    template <class _Rep, class _Period>
121    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
122    bool try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time)
123    {
124        if (__rel_time == chrono::duration<_Rep, _Period>::zero())
125            return try_acquire();
126        auto const __test_fn = [this]() { return try_acquire(); };
127        return std::__libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
128    }
129    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
130    bool try_acquire()
131    {
132        auto __old = __a_.load(memory_order_acquire);
133        while (true) {
134            if (__old == 0)
135                return false;
136            if (__a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
137                return true;
138        }
139    }
140};
141
142template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
143class counting_semaphore
144{
145    __atomic_semaphore_base __semaphore_;
146
147public:
148    static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");
149
150    static constexpr ptrdiff_t max() noexcept {
151        return __least_max_value;
152    }
153
154    _LIBCPP_HIDE_FROM_ABI
155    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count)
156    {
157        _LIBCPP_ASSERT_UNCATEGORIZED(
158            __count >= 0,
159            "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
160            "initialized with a negative value");
161        _LIBCPP_ASSERT_UNCATEGORIZED(
162            __count <= max(),
163            "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
164            "initialized with a value greater than max()");
165    }
166    ~counting_semaphore() = default;
167
168    counting_semaphore(const counting_semaphore&) = delete;
169    counting_semaphore& operator=(const counting_semaphore&) = delete;
170
171    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
172    void release(ptrdiff_t __update = 1)
173    {
174        _LIBCPP_ASSERT_UNCATEGORIZED(__update >= 0, "counting_semaphore:release called with a negative value");
175        __semaphore_.release(__update);
176    }
177    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
178    void acquire()
179    {
180        __semaphore_.acquire();
181    }
182    template<class _Rep, class _Period>
183    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
184    bool try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time)
185    {
186        return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
187    }
188    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
189    bool try_acquire()
190    {
191        return __semaphore_.try_acquire();
192    }
193    template <class _Clock, class _Duration>
194    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
195    bool try_acquire_until(chrono::time_point<_Clock, _Duration> const& __abs_time)
196    {
197        auto const __current = _Clock::now();
198        if (__current >= __abs_time)
199            return try_acquire();
200        else
201            return try_acquire_for(__abs_time - __current);
202    }
203};
204
205using binary_semaphore = counting_semaphore<1>;
206
207_LIBCPP_END_NAMESPACE_STD
208
209#endif // _LIBCPP_STD_VER >= 14
210
211_LIBCPP_POP_MACROS
212
213#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
214#  include <atomic>
215#endif
216
217#endif //_LIBCPP_SEMAPHORE
218